~ Common Emitter Amplifier ~ Transistor r_bb Calculator ~

To be used with a Common Emitter test jig as described Here

Boltzmann Constant — k	1.38062×10-23	J/K    [ Joules/Kelvin ]
Charge on a single Electron — q	1.60218×10-19	C   [ Coulomb or Amperes/sec ]
Measurement device load — RL		Ω Parallel with RC assumed a.c. coupled
Collector Resistor — RC		Ω Load resistor  [ RE & RS assumed = 0 ]
Measured Voltage across RC — VRC		V  gives  IC = VRC/RC =  mA
Measured Voltage Gain		AVm  [ affected by high rbb and low ro ]
Noise Voltage at Collector — VNtot		µVrms  Enter Measured value @ IC T & Bn
Transistor d.c. current gain — β		hFE from data sheets @ IC or measured
Device Temperature — T		˚C    Calculation uses  T =   ˚K
Measurement Noise Bandwidth — Bn		Hz — Is not the −3dB BW see Here
Calculated results using the Variables entered above
Voltage Gain neglecting ro — AV		V/V = α RC/re
Emitter Internal Resistance — re		Ω = VT/IC  VT= kT/q =   mV
Load resistor including ro — RCm		Ω       RC∥ RL =   Ω
Internal Collector Resistor — ro		kΩ     VA = ro · IC =   V
Noise at Collector due to IC — VNIC		µVrms = √2 q IC Bn  × (RCm−re)
Noise at Collector due to RCm — VNRC		µVrms = √4 k T Bn RCm
Noise at Collector due to rbb		µV = √V²Ntot−V²NRC−V²NIC = √V²Nrbb+V²NIB
rbb calculated from √V²Nrbb+V²NIB		Ω    The −ve result

 

The value of R_Cm [ R_C modified by r_o ] must be less than the total collector load R_C∥R_L ~ The calculated value of r_bb must be positive and the negative result negative for the answer to be plausible ~ NaN [Not a Number] may indicate the measured gain or  some other inputs are not correct or a non numeric input was entered

Some transistor parameter testers of the past used to derive data sheet Noise Voltage Referred to Input or rbb directly had resistance in the Base–Emitter path that made rbb effectively up–to 10Ω or more higher ~ See rbb calculated from data–sheet values  but check the data–sheet noise units and the bandwidth used and if possible the equipment used to make the measurement

 

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